Study On The Properties And The Mechanism Of Rare Earth Modified Titanium Dioxide Photocatalyst

Titanium dioxide is a widely used semiconductor, because of its high catalytic activity, chemical stability, non-toxicity, low-cost and etc., it is considered to be the most important photocatalyst, and has been widely used in the fields of clean energy and environmental protection. However, the wide bandgap （3.2 eV） of TiO₂ leads to its lower utilization of sunlight, which greatly limits the photocatalytic application of TiO₂. Under the consideration of the contact area of catalyst and pollutants in the application, TiO₂ photocatalyst are usually formed into nano-powder. However, the hard recovery and separation from the water are resulted from the decrease of TiO₂ particle sizes. This work mainly focuses on the low photoquantum yield and utilization rate of visible light of TiO₂. Rare Earth （RE）, which embodies unique 4f electronic configuration, and non-metallic elements are adopt to dope and modify TiO₂, and a variety of different doped-TiO₂ photocatalysts have been prepared through the sol-gel method. This thesis presents a systematic study of structural property, optical property, electronic property and catalytic performance of doped-TiO₂. Meanwhile, the preliminary exploration and research of doping TiO₂’s reutilization have carried out in this paper.La, Ce, Eu and Y have been used to dope the TiO₂, structural characterization results indicate that all the different RE doping have the effect of refining TiO₂ grain. And the morphology of all RE doping TiO₂ is flake structure. As the ionic radius of RE is greater than Ti, their doping introduce the lattice expansion of TiO₂. All the RE doping results in the light absorption range of TiO₂ enlarge to the visible light area, and enhances the visible light utilization to the different degree at the same time. La、Ce、Eu and Y doped TiO₂ all have better degradation efficiency of methylene blue （MB） both under visible and ultraviolet light, and the degradation rate improvement of RE-doping under ultraviolet light is better than that of visible light. Under the best doping level of each element, the catalytic enhancement of La and Ce are greater than that of Eu and Y.On the basis of La or Ce single doping, and further import Boron （B）, which is a small radius and chemical active non-metallic element, as the second doping element into TiO₂. The RE-B co-doped TiO₂ are all anatase and their morphology transform from flake to granular. The grain diameter of RE-B-TiO₂ are about 10nm, and their absorption edge appear obvious red shift, the absorption edge of La1.5-B20-TiO₂ moves to 466nm, equivalently the forbidden band of La1.5-B20-TiO₂ reduce 0.4eV compared with pure TiO₂. The PL spectra show that the carrier recombination probability of RE-B co-doped TiO₂ is significantly lower than other samples. XPS detect B is doped into the TiO₂ lattice gap, the chemical state of B exists in the form of Ti-O-B in La-B-TiO₂, and La exists in the form of Ti-O-La. There are two kinds of chemical state of B in Ce-B-TiO₂:a part of being TiB2 and another part replace O enter into TiO₂ crystal lattice. The chemical forms of Ce in Ce-B-TiO₂ coexist with Ce3+ and Ce4+, a part of Ce replace Ti enter into TiO₂ in the form of Ti-O-Ce, and another part forms CeO₂ cover on TiO₂ particals. When the molar percentage of La to Ti and B to Ti are 1.5 and 20, the La-B TiO₂ achieve the best catalytic activity, the degradation rate to MB is 80.67% and 74.78% under ultraviolet and visible light separately, which is 2.7 and 1.7 times compared with pure TiO₂ respectively.Fluorine （F） is adjacent to O in the system periodic, and the radius of fluorine is very similar to O. Many differents La:Ti and F:Ti ratio La-F co-doped TiO₂ have been prepared through co-doping and two-step co-doping. The different doping mode almost makes no difference to the crystal of product, particle shape and light response range. All the La-F co-doped TiO₂ are anatase and their morphology are similar globular shape, however the two-step La-F co-doping TiO₂ behave more aggregate and integrate because of undergoing high temperature more times. F exists in two forms in La-F-TiO₂:one part absorbs on the catalyst surface and another part replace O enter into TiO₂ crystal lattice, while strong electronegativity of F leads to the binding energy of Ti shift to the higher direction. The degradation rate of all the La-F co-doping TiO₂ to MB are above 90% under visible light within 120min, while the two-step La-F doping samples could also degrade MB rapidly and the degradation rate could reach more than 96% within 120min.In order to probe into the inner reason of two-step La-F doping TiO₂’s excellent catalytic activity, electrochemical impedance spectroscopy （EIS） is adopt to proceed the electrochemical testing for La-F-TiO₂ electrode. Take the results of capacitance fitting from the equivalent circuit, solve for carrier concentration and flat-band potential of electrode according to Mott-schottky equation. The result shows that the flat-band potential shift to the poaitive direction and the carrier concentration increase along with the doping La amount increased. Form an N-N heterojunction inside the two-step co-doping TiO₂ is the internal cause of high catalytic activity and carrier concentration. Different Fermi level of two kinds semiconductor reduce the formation of build-in internal electric in the TiO₂ electrode, the build-in internal electric can act as the driving force for the carrier mobility, and result in the effectively separation of carrier and improving of the catalytic activity finally.Ternary phase diagram method has been adopted to produce the high Al13 content Poly Aluminium Chloride （PAC） flocculant. The optimal flocculant dosage is 10mg/L for the recovery of doping TiO₂. The degradation rate of La1.5-TiO₂,La1.5-B20-TiO₂,N-La1.0-F10-TiO₂ and La1.5-F10-TiO₂ to MB declines significantly with the number of recycling increases. Under ultraviolet and visible light, the degradation rate of MB dropped to around 30% and catalyst deactivation by more than 60% after 6 cycles, while the catalytic activity can keep more than 70% within 3 cycles. The decline of photocatalytic capability is closely related to the morphology structure of photocatalyst. The more evenly the photocatalyst distributed, more looser it organized and bigger surface it has, the more the degradation rate decreased when it been reused. The main reason for the decreasing in degradation rates is the particle agglomerates and decreasing of surface area of doped TiO₂.